Morph Targets are a way to store a geometry configuration so that you can recall it later. You can only create one Morph Target per SubTool at any one time and it will be linked to the subdivision level on which is created. Morph Targets require the number of points to remain static, so geometry can’t be deleted or added without destroying the Morph Target.
A typical use for Morph Targets is for restoring an original base mesh immediately prior to displacement or normal map generation.
Morph Targets can also be used as part of the sculpting process, such as creating some asymmetry by restoring parts of a stored sculpt using the Morph brush.
For the Morph Target controls see the Tool >Morph Target sub-palette.
For storing pose or mesh variations consider using 3D Layers.